When combining or fusing two digital images, conventional methods combine a weighted image-A with a weighted image-B. This is generally the least computationally complex approach to fusing image data. The resultant image may subsequently be processed by various image enhancement steps, such as dynamic range adjustment (contrast enhancement) and tonal transfer curve (gamma) adjustment to provide a visually appealing image. Prior to combining the imagery, both images may be contrast enhanced to maximize detail and edge content. Tonal corrections may also be performed.
It is typically assumed that if the two incoming images (A and B) are optimally enhanced prior to mixing, then the resulting image also includes high information content. In practice, however, this is not realized, because the weighted images tend to produce ‘clumped’ output data sets. When fusing each possible combination of input gray level pairs, the output distribution of the fused dataset typically includes more data in the central range than at the outer edges. The ‘clumped’ output may be seen in FIGS. 1A, 1B and 1C, in which histograms of output gray levels resulting from combining each possible input pair of gray levels from image A and image B are shown. FIG. 1A results from combining all possible input gray level pairs, using a mix-level of 0.25; FIG. 1B results from combining all possible input gray level pairs using a mix-level of 0.5; and FIG. 1C results from combining all possible input gray level pairs using a mix-level of 0.75.
The resulting output using the ratio-metric approach shown in these figures achieves images with characteristic middle-gray weighted data, usually low contrast data, which are not visually appealing. Furthermore, determining an appropriate gamma, or TTC curve, and a contrast stretch method to enhance these image data sets—regardless of input content—is not straight forward.
The present invention, on the other hand, targets an output distribution of a combined image, in which the look and tone of the fused brightness of the image are controlled. As will be explained, the present invention provides a fixed distribution look-up-table (LUT) for combining two digital images to produce a fused image that maintains expected contrast and does not reduce it. The present invention also provides an algebraic approach to obtaining a fixed distribution, when fusing two images, and achieves the fusion without need of a LUT that requires large memory storage and large processing power.